Modern mobile communication technologies tend to provide multi-media services with a high transmission rate. FIG. 1 is a system architecture evolution (SAE) system architecture.
In FIG. 1, user equipment (UE) 101 is a terminal device that receives data. Evolved universal terrestrial radio access network (E-UTRAN) 102 is a radio access network, which includes a macro base station (eNodeB/NodeB) that provides a radio network interface for the UE. Mobility management entity (MME) 103 is responsible for managing mobility context, session context and security information of the UE. Serving gateway (SGW) 104 is mainly responsible for providing a user plane, and the MME 103 and the SGW 104 may be located in a same physical entity. Packet data network gateway (PGW) 105 is responsible for functions such as charging, lawful interception, etc., and it may also be located in a same physical entity with the SGW 104. Policy and charging rules function (PCRF) 106 provides QoS policies and charging rules. Serving GPRS support node (SGSN) 108 is a network node device that provides routing for data transmission in a universal mobile telecommunications system (UMTS). Home subscriber server (HSS) 109 is a home sub-system of the UE, responsible for protecting user information such as a current location of the user equipment, an address of a serving node, user security information, packet data context of the user equipment, etc.
3GPP raises requirements of small cell enhancements in release 12 (Rel-12). Targeted scenarios of small cell enhancements include scenarios with macro cell coverage and without macro cell coverage, indoor and outdoor scenarios, and enhancements for ideal and non-ideal backhaul, as shown in FIG. 2.
In the circumstance that there is macro cell coverage, a carrier aggregation technology between different base stations may be deployed. A Macro cell and a small cell may work at different frequency bands. There are two kinds of architectures when using the carrier aggregation technology between different base stations, namely radio-access-network (RAN) based separate architecture and core-network (CN) based separate architecture of user plane data. Core-network based separate architecture means that, for bearers that are set up in a pico cell, data is directly sent to the pico cell by a SGW of a core network, and a user plane will not be forwarded by a macro cell.
In the small cell architecture, a UE can send/receive data at two base stations at the same time, which is called dual-connectivity. One of the base stations, which is responsible for sending radio resource control (RRC) messages to the UE and is responsible for interaction with a core network control entity, is called a master base station, MeNB, and the other base station is a secondary base station, SeNB. There is a cell at the master base station for the UE, which is a primary cell of the UE, Pcell, and RRC messages are sent to the UE via the primary cell, other cells are secondary cells, Scells. Among Scells of the secondary base station, a Scell is a primary cell of the secondary base station, pScell (having functions of a pScell). There are uplink physical layer control channels in the pScell, and there is no uplink physical layer control channel in other Scells. A cell group of the master base station is a master cell group (MCG) and a cell group of the secondary base station is a secondary cell group (SCG).